Adaptive fragmentation for wireless network communications

ABSTRACT

A wireless local area network (LAN) adapter ( 20 ) that optimizes the length of message packets, for example according to the IEEE 802.11 standard, and in an environment having interfering transmissions (BL 1  et seq.), is disclosed. The disclosed adapter ( 20 ) executes an adaptive process by way of which an adjustment to the packet length is derived based upon rate measures for the most recent two trial packet lengths. The rate measure corresponds to a packet success rate for that packet length, determined either from an estimating function or by actual measurements, multiplied by a ratio of the data portion of each packet to a total packet length including interpacket spacing. Upon convergence as the adjustment becomes smaller, the optimized packet length for best data rate given the present interference. A method of determining the need for packet length optimization is also disclosed, in which the actual packet error rate is compared against an expected packet error rate based upon signal-to-noise ratios.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority, under 35 U.S.C. §119(e), ofprovisional application No. 60/262,507, filed Jan. 18, 2001.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] This invention is in the field of wireless communications, and ismore specifically directed to the wireless transmitting of data packetsin an environment containing possible interfering communications.

[0004] Wireless local area networks (LANs) have become increasinglypopular in recent years. Typically, wireless LAN installations includean access point sited within the vicinity of the various clientworkstations. The access point, which is typically a network elementcoupled to a computer workstation by way of Ethernet cabling or thelike, serves as a hub for wireless devices within its communicationsrange. Bidirectional communications are carried out between the accesspoint and wireless network-enabled devices that are in range (typicallyon the order of 100 m), enabling the wireless devices to communicatewith one another, with other computers resident on the same wirednetwork as the access point, and with remote computers over theInternet.

[0005] Under current wireless networking technology and standards, anexample of such being the IEEE 802.11b standard, the wirelesscommunications are packet-based, in that each transmission istransmitted in the form of multiple packets. By being packet-based, thepackets need not be transmitted or received in sequence, and willgenerally not be contiguous in time. Indeed, as known in the art,packets that are corrupted in transmission are retransmitted later intime. Upon receipt of all of the packets for a communication, thereceiver resequences and combines the packets into a coherent message.In the 802.11 context, each message packet typically includes a preambleand header portion that contains control information and alsoinformation identifying the packet (identifying the message, thesequence of the packet in the message, source and destination nodes,etc.), and also includes a payload portion that contains the actual databeing communicated, along with a checksum by way of which errors in thepayload portion can be detected and possibly corrected.

[0006] Modern wireless networks typically operate in the unlicensedIndustrial, Scientific, and Medical (ISM) band which, as known in theart, includes frequencies from about 2400.0 MHz to about 2483.5 MHz.Conventional 802.11 transmission s are signals according to the QPSK andBPSK constellations that are modulated into a “channel” within the ISMband having about a −20 dB bandwidth of about 16 MHz, and providing datarates that can reach up to about 11 MHz. Other wireless devices alsocommunicate in this band. An example of such devices are thenewly-popular “Bluetooth” devices, which transmit in a frequency-hoppingmanner within the ISM band. More specifically, Bluetooth transmissionsare carried out in channels that are about 1 MHz in width (−20 dB) thatchange frequency periodically (e.g., about every 625 μsec).

[0007] Considering the likelihood that both 802.11 and Bluetooth devicesmay be operating within the range of the 802.11 access point, and alsoconsidering other ISM transmissions such as wireless telephones, garagedoor openers, and the like, signal interference can often occur. If twodifferent transmissions occur at the same time in the same frequencychannels within the ISM band, typically both transmissions will becorrupted. Accordingly, those in the art have studied ways to reduce theincidence of collisions in this unlicensed band.

[0008] Fragmentation is a conventional approach to reducing the packeterror rate due to interference. In general, fragmentation enforces anupper limit on packet length, thus reducing the likelihood that aninterfering signal will occur within the packet. Typically, under the802.11b standard, a parameter is used to set the number of payload databytes transmitted in a packet for a given bit rate, which thus sets thepacket length (or fragmentation level). As is fundamental in the art,because interference along any portion of the packet will corrupt theentire packet, longer packet lengths generally result in a higherprobability of packet error due to interference, for a given level ofinterference. Increasing the fragmentation of the transmission, by usingshorter payload portions in each packet, therefore provides a reducedpacket error rate. However, because of the existence of a certain amountof overhead associated with the transmission of each packet the averagethroughput rate decreases as the packet lengths are decreased. Examplesof such network overhead include packet preambles , packet headers andspacing between packets.

[0009] This tradeoff between packet error rate and overhead makes theselection of a fragmentation in a wireless network an importantconstraint on the overall network performance. Making this selectioneven more difficult is the nature of the interferers that are nowcommonly present within a wireless network signal range. Many potentialinterferers, such as wireless telephones, garage door openers, and thelike, may interfere only within certain times of operation. Otherinterferers, such as frequency-hopping transmissions in a Bluetoothnetwork, further complicate the fragmentation selection, considering theephemeral nature of the transmissions in the various channels. It cantherefore be quite difficult to select a fragmentation level or atransmission channel to maximize the average throughput rate.

BRIEF SUMMARY OF THE INVENTION

[0010] It is therefore an object of this invention to provide a methodand a network element for wireless communication of digital data inwhich the average throughput rate is maximized.

[0011] It is a further object of this invention to provide such a methodand element in which the optimizing of the average throughput rate canadapt to the environment in the frequency band being used.

[0012] It is a further object of this invention to provide such a methodand element for detecting a condition in which packet error rates arepredominantly due to interference.

[0013] It is a further object of this invention to provide such a methodin which the optimization can be effected at the transmitter, withoutrequiring change in the characteristics of the receiver.

[0014] Other objects and advantages of this invention will be apparentto those of ordinary skill in the art having reference to the followingspecification together with its drawings.

[0015] This invention may be implemented by way of an adaptiveoptimization of the packet length for wireless transmissions,particularly in environments with interferers. According to theinvention, a successful rate value is determined for an iteration of apacket length. For example, the successful rate value may correspond tothe product of the probability of communicating a packet without error,with a ratio corresponding to the payload length over the totaltransmission time for the packet. The next packet length iteration isthen determined by applying a weighted difference of the most recentsuccessful rate values to the current packet length iteration.

[0016] According to another aspect of the invention, a method ofdetermining the effects of interference on packet error length involvesthe comparison of an expected packet error rate based on signal-to-noisecharacteristics of the channel with an actual packet error rate based onerror detection at the receiver for the actual payload.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0017]FIG. 1 is an electrical diagram, in block form, of a wirelesslocal area network (LAN) in an environment with interferingtransmissions such as from a group of Bluetooth devices.

[0018]FIG. 2 is an electrical diagram, in block form, of a wirelessnetwork adapter within which the preferred embodiment of the inventionmay be implemented.

[0019]FIG. 3 is a flow chart illustrating a method of determining thepresence of packet error due to interference, according to the preferredembodiment of the invention.

[0020]FIG. 4a is a qualitative plot of packet success rate versus packetlength, under various conditions.

[0021]FIG. 4b is a qualitative plot of data rate versus packet lengthfor one of the conditions of FIG. 4a.

[0022]FIG. 5 is a timing diagram illustrating the definition of varioustimes within the transmission of a packet, according to the preferredembodiment of the invention.

[0023]FIG. 6 is a flow chart illustrating a method of optimizing packetlength in an interfering environment, according to the preferredembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] Referring now to the Figures, an exemplary implementation of thisinvention in connection with a wireless local area network (LAN) willnow be described. As will become apparent, this invention isparticularly beneficial when applied to wireless networks, consideringthe increased average data throughput rate that is achievable over awide variety of time-varying conditions, by way of this invention. Thoseskilled in the art having reference to this specification will readilycomprehend, however, that this invention may also be used in connectionwith other packet-based communications applications, with particularbenefit to those applications that have ephemeral and frequency-hoppinginterferers. Accordingly, it is contemplated that those skilled in theart will recognize that the following description is presented by way ofexample only.

[0025]FIG. 1 illustrates an example of a wireless LAN environment intowhich the preferred embodiment of the invention is implemented. As isfundamental to those in the art, wireless LAN environments can varywidely from installation to installation, and indeed can vary widelyover time within a single installation as different devices areinstalled, used, or enter and exit the signal range of the wireless LAN.Accordingly, the environment of FIG. 1 is presented by way of exampleonly.

[0026] In FIG. 1, computer 2 is enabled to carry out wireless LANcommunications to and from wireless LAN access point 10 by way ofwireless LAN adapter 20, which transmits and receives signals accordingto the IEEE 802.11 standard over wireless link WL1. FIG. 2 illustratesan example of the construction of wireless LAN adapter 20, incooperation with computer 2.

[0027] In the example of FIG. 2, wireless LAN adapter 20 includes hostinterface 22, which controls communications with computer 2 over busPCI. Host interface 22 communicates with medium access controller (MAC)25, which is a conventional controller known in the art. Embedded CPU 23and off-chip memory 24 cooperate with MAC 25, to effect control ofadapter 20 and to provide additional program memory, respectively.Physical layer (PHY) device 26, also referred to as a basebandprocessor, performs conventional modulation of data signals from MAC 25into a spread spectrum form for transmission via radio circuitry 27, anddemodulation of spread spectrum signals received from radio circuitry 27into baseband. Antenna A is coupled to radio circuitry 27, in theconventional manner. In this example, interface 22, MAC 25, PHY 26, CPU23, and some or all of memory 24 may be implemented into a singleintegrated circuit, such as the ACX100 spread spectrum processor withmedium access control, available from Texas Instruments Incorporated. Ofcourse, other architectures may alternatively be used for wireless LANadapter 20. The arrangement of FIG. 2 is presented merely by way ofexample.

[0028] Computer 2 is also connected to its various input and outputdevices by way of different facilities. For example, computer 2 ishardwire connected to its monitor 4 by a conventional VGA cable.Computer 2 is also in wireless communication with printer 5, keyboard 6,and mouse 7, by wireless links BL1, BL2, BL3, respectively; in thisexample, links BL1 through BL3 effect communications according to theBluetooth standard, which is a well-known short range frequency-hoppingcommunications standard.

[0029] The wireless LAN of the example of FIG. 1 also includes otherworkstations and network services. In this example, workstation 8 isalso connected to access point 10 by 802.11 wireless link WL2 via awireless LAN adapter (not shown), while server 12 is hardwire connectedto access point 10 by an Ethernet or other facility. Workstation 8 alsoshares printer 5, by way of Bluetooth wireless link BL4. In thisexample, server 12 is connected to modem 14, by way of which Internetaccess is provided to server 12, and also to computer 2 and workstation8 over wireless links WL1, WL2, respectively.

[0030] As evident from the example of FIG. 1, the multiple wirelesscommunications provide significant opportunities for interference. The802.11 wireless links WL1, WL2 must, of course, be coordinated so as tonot interfere with one another; typically, access point 10 controls thechannels (in frequency, or alternatively in time-multiplexed fashion)assigned to these links to avoid interference. However, non 802.11wireless activity, such as Bluetooth wireless links BL1 through BL 3,cannot be controlled in frequency or time by access point 10. Rather,according to this preferred embodiment of the invention, the presence ofthese interferers is detected and, in that event, the packet lengths ofthe wireless LAN links WL1, WL2 are optimized for maximum data rate.

[0031] As discussed above in the Background of the Invention, thelikelihood that a packet will be interfered with increases withincreasing packet length; conversely, if one assumes no interference,the transmission data rate will decrease with decreasing packet lengthbecause of packet overhead. According to this invention, a peak datarate as a function of packet length can be derived for a giveninterference probability, as will now be described.

[0032]FIG. 3 illustrates the construction of a message packet, such asmay be transmitted and received according to the IEEE 802.11 standard.The packet includes preamble PR, which includes control and otherinformation regarding the channel, specifically including a currentmeasurement of the signal-to-noise ratio (SNR) andsignal-to-interfering-noise ratio (SINR) under the 802.11 standard. Eachpacket also includes header H, which contains information identifyingthe message with which the packet is associated, the sequence of thispacket in that message, the source and destination addresses of thepacket, the packet length, and the like. Payload portion PL of coursecontains the data being transmitted in the packet, and includes CRCchecksum by way of which the receiver detects, and possibly corrects,one or more bit errors in payload portion PL.

[0033] By way of definition, the duration of preamble PR and header Hportions of the packet occupies time t_(H), and is considered to befixed among all packets. Payload portion PL, including its CRC checksum,has a duration t_(D), and will be varied, according to the preferredembodiments of the invention, for optimum effective data rate. As knownin the art, the 802.11 standard (as well as other standards), enforce aminimum interpacket spacing t_(O), which will also be treated as a fixedquantity in this description. By way of definition, packet length t_(p)will refer to the sum of times t_(H) and t_(D). It will be understood bythose skilled in the art having reference to this specification that thevarious times within the packet may be defined differently, and used inoptimizing the packet length, while not departing from the spirit ofthis invention.

[0034]FIG. 4a illustrates various possible shapes of a success functionq, which is the probability that a packet is transmitted without errorin a given environment that includes interfering transmissions, as itvaries with packet length t_(p). As shown in FIG. 4a, a minimum packetlength t_(p) exists, corresponding to a packet having one byte ofpayload portion PL in combination with the preamble PR and headerportions H; this minimum of course will be briefly (one byte) longerthan time t_(H). As discussed above, the success rate is maximum for theshortest packets, considering that the probability of interference witha packet is lower as the packet size decreases. The particularrelationship of success function q with increasing packet length t_(p)may take various forms, as suggested by FIG. 4a, including linear andhigher order forms. While it is contemplated that a linear approximationq′ will usually resemble this relationship, the present invention isequally applicable to other orders of success function q.

[0035] For a given success function q(t_(p)), one can derive theeffective data rate R as a function of packet length t_(p):$R = {{\frac{t_{D} \times r}{t_{H} + t_{D} + t_{O}} \times {q( t_{p} )}} = {\frac{( {t_{p} - t_{H}} ) \times r}{t_{p} + t_{O}} \times {q( t_{p} )}}}$

[0036] where r is the bit rate of data transmission in payload portionPL of the packet. For the case of linear success function q′ of FIG. 4a,the data rate function R′ as a function of packet length t_(p) isplotted in FIG. 4b. As evident from FIG. 4b, data rate function R′ has asingle maximum. According to the preferred embodiment of the invention,an adaptive approach is performed to determine this maximum inoperation, and to set the packet length t_(p) for each transmission toachieve this maximum.

[0037] It is possible that no interferers are in the vicinity ofwireless LAN adapter 20 and its fellow LAN elements. Knowledge ofwhether or not an inteferer is in the vicinity may be useful for variouspurposing such as to notify a user, to provide input to interferenceavoidance algorithms, to assist networks administrators and to determinewhether or not adaptive fragmentation is useful due to the presence ofinterference. According to the preferred embodiment of the invention,therefore, a method for determining whether interference is presentwould be useful. This detection method will now be described withreference to FIG. 5.

[0038] The method of FIG. 5 and the other operations of LAN adapter 20described in this specification are preferably executed by programmablelogic within LAN adapter 20, for example by embedded CPU 23. Of course,if MAC 25 or baseband processor 26 has sufficient processing capacity,those other devices may instead be used to perform these operations.Further in the alternative, while the described examples of the methodsof this invention are realized by way of computer program routinesexecutable by programmable logic, it is also contemplated that theseoperations could be realized by custom hardwired logic, if desired.

[0039] Referring to FIG. 5, measured values of signal-to-noise ratio(SNR) and signal-intersymbol interference-noise ratio (SINR) arereceived by LAN adapter 20 in process 30. As noted above, theseparameters are estimated during reception of packet preamble PR, fromanother network element in the wireless LAN. Each communicationschannel, being bidirectional, is evaluated by the network elements onthat channel, and the network elements may communicate theirmeasurements of channel behavior to one another. In the case of the SNRand SINR measurements, the various network elements in the network areable to determine these basic performance parameters by analysis of theknown contents of packet preamble PR, as is known in the art. As knownin the art, the SNR relates to the signal strength and noise within agiven channel, while the SINR relates to the intersymbol interferenceexperienced, for example because of multipath interference. These valuesthus are not intended to account for the presence of other interferingtransmissions within the network range.

[0040] Once the SNR and SINR values for the current channel are known,LAN adapter 20 calculates an expected packet error rate p_(e) in process32. Methods for the statistical determination of the probability that agiven packet will fail, for a given set of SNR and SINR values, areknown in the art. As noted above, however, this expected packet errorrate p_(e) will not fully appreciate the effects of interferingtransmissions, such as may be present due to Bluetooth or other sources.

[0041] In process 34, LAN adapter 20 estimates the true packet errorrate p, including the effects of interfering transmissions. Thisderivation is preferably performed for a channel by analyzing the numberof packets that it transmits and that are not safely received, relativeto the total number of packets transmitted over that channel. Thecriteria used to determine successful transmission is preferably basedon the CRC checksum that is transmitted with the payload of each packet,and that the receiving network element, such as access point 10 for LANadapter 20, checks against the payload portion PL of the packet itself.The overall results of the fraction of those packets that were receivedsafely can be derived from acknowledgement messages from access point10. Alternatively, the network elements in the wireless LAN maycommunicate the success ratio for each channel by way of controlpackets. In any event, whether LAN adapter 20 calculates the packeterror itself from acknowledge messages, or receives a packet error ratevalue from access point 10 or elsewhere in the network, the actualpacket error rate p is determined in process 36.

[0042] In decision 36, LAN adapter 20 determines whether the actualpacket error rate p exceeds the expected packet error rate p_(e) by morethan a threshold amount ε. If no interfering transmissions are present,the actual packet error rate p will vary about the expected error ratep_(e), either due to the approximation of the expected error rate p_(e)or because of statistical variations. As such, threshold ε is preferablyset high enough so that, when exceeded, one can be confident thatinterfering transmissions are actually present in the networkenvironment. If threshold ε is not exceeded (decision 36 is NO), controlreturns to process 30 to continue the monitoring of the packet errorrates. If interfering transmissions appear to be present, based on thecomparison of the expected error rate p_(e) and the actual packet errorrate p, LAN adapter 20 then detects the presence of an interferer, suchas Bluetooth, that was not anticipated by the SNR and SINR, in process40.

[0043] It is contemplated that the process of FIG. 5 may be used todetermine whether interfering transmissions are present in the network,in conjunction with a wide variety of optimization processes, includingprocess 40 according to the preferred embodiment of the invention aswill be described in detail below. Alternatively, once interference hasbeen detected using the method of FIG. 5 according to the preferredembodiment of the invention, other conventional methods for avoiding theeffects of interference may also be used, such methods including dynamicchannel selection, conventional fragmentation, and the like.

[0044] Referring now to FIG. 6, packet length optimization process 40according to the preferred embodiment of the invention will now bedescribed in detail. While process 40 is preferably used in combinationwith the interference detection process of FIG. 5 discussed above, it isalso contemplated that process 40 may be used with other means ofdetecting interference, or alternatively may be unconditionally usedupon the establishment of each and every communications channel.

[0045] In process 42, learning constant μ is initialized. As will becomeapparent below, learning constant μ is a constant that determines therate of change of packet length t_(p) for a given calculated data ratechange. The value of learning constant μ is preferably determinedempirically for the expected wireless LAN environment, considering thata value of learning constant μ that is too high will result inoscillation, while a value that is too low will delay optimization.According to this example, a useful value of learning constant μ isabout 1.275. In process 44, other parameters in optimization process 40are initialized, including the appropriate values of header and preambletime t_(H) and interpacket spacing t_(O). In process 46, an initialpacket length t_(p,k) and a prior initial packet length t_(p,k−1) areset; in addition, a prior rate measure F_(k−1) may be set (to anyarbitrary value, including zero).

[0046] In process 48, a current packet success rate q′(t_(p,k)) forinitial packet length t_(p,k) is determined. Measurement process 48 maybe performed by the transmission of actual message packets at thecurrent packet length t_(p,k) and the calculation of an actual packetsuccess rate based on checksum results at the receiver, as describedabove relative to process 34 (FIG. 5). Alternatively, a success ratefunction q′(t_(p)) may be known or assumed, in which case the packetsuccess rate q′(t_(p,k)) is determined by applying the current packetlength t_(p,k) to this function.

[0047] In the adaptive algorithm of optimization process 40, a ratemeasure F_(k) is next derived, for the current packet length t_(p,k).This rate measure F_(k) operates as a fitness function, in the adaptivealgorithm sense, by way of which the packet length t_(p,k+1) for thenext iteration is determined. According to this preferred embodiment ofthe invention, an example of rate measure F_(k) is:${- F_{k}} = {{q^{\prime}( t_{p,k} )} \times \frac{( {t_{p,k} - t_{H}} )}{( {t_{p,k} + t_{O}} )}}$

[0048] As evident from this definition of rate measure F_(k), theparameters involved in the calculation of process 50 includes thesuccess rate (estimated or actual) for the current packet lengtht_(p,k), and a ratio of the actual data portion of the packet relativeto the entire packet length, including its interpacket spacing. Otherfactors may also be used in this measure, including maximum data rateand the like, as desired.

[0049] In process 52, LAN adapter 20 derives difference value Δ based onthe difference between the current and previous rate measures F, asfollows:

−Δ=F _(k) −F _(k−1)

[0050] The negative applied to rate measure −F_(k) and difference valueΔ reflects the use of these values as negative feedback in theadjustment of the packet length t_(p) in process 54. In process 54, thenext packet length t_(p,k+1) is generated based on the difference Δ inthe rate measures F, with the learning factor μ, as follows:

t _(p,k+1) =t _(p,k)+μΔ

[0051] Decision 55 is then performed, to determine if additionaladjustment of the packet length by the execution of another iteration ofthe loop is necessary. The decision criterion used in decision 55 may bea convergence criterion, by way of which the difference between nextpacket length t_(p,k+1) and prior packet length t_(p,k) is measuredagainst a convergence threshold; alternatively, optimization process 40may be performed for a preselected number of iterations, in which casedecision 55 would simply interrogate a loop counter. In any event, ifthe appropriate convergence criterion has not yet been met (decision 55is YES), index k is incremented in process 56 so that the next packetlength becomes the current packet length for purposes of this method,and control passes to process 48 for measurement or determination of thepacket success rate q′.

[0052] On the other hand, upon the convergence criterion being reached(decision 55 is NO), the next packet length t_(p,k+1) is then applied totransmissions from LAN adapter 20 over that channel. In the case of802.11 communications, the adjustment of packet length t_(p) is made byadjusting a parameter D_(B) which specifies the number of data bytes tobe transmitted in each packet; the data time t_(D) will correspond tothe data byte parameter D_(B) times the channel data rate r. Thisoptimized packet length t_(p,k+1) is thus the packet length for whichthe actual data rate is maximized, for the interference present in thenetwork. With reference to FIG. 4b, the optimized packet lengtht_(p,k+1) is illustrated as corresponding to that for which the datarate R is at approximately a maximum. This is accomplished, according tothis embodiment of the invention, by iterating the process of FIG. 6until the difference in rate measure F nears zero, corresponding to thederivative of the rate curve R being at or near zero at this maximum. Ifdesired, following convergence, the method of FIGS. 5 and 6 may berepeated periodically, to ensure that the data rate remains optimized asinterference conditions in the network area change over time.

[0053] This invention thus provides important advantages in theoperation and use of a wireless network, in environments in which thenetwork is prone to interference from other wireless devices. Theeffects of interference in causing packet errors are minimized byfragmentation of the messages, but in an adaptive manner that maximizesthe overall successful packet data rate. In addition, this inventionprovides a way in which the presence of interference can be detected,and the optimization process initiated or repeated, as necessary. Inaddition, the receiving network element need not perform any additionalfunction in order for the optimization to be executed; rather, thetransmitting network element can optimize the packet length using onlyfeedback from the access point or other receiving network element thatis already provided under the appropriate standard, such as IEEE 802.11.Of course, this invention may also be used in connection with otherwireless standards.

[0054] While this invention has been described according to itspreferred embodiments, it is of course contemplated that modificationsof, and alternatives to, these embodiments, such modifications andalternatives obtaining the advantages and benefits of this invention,will be apparent to those of ordinary skill in the art having referenceto this specification and its drawings. It is contemplated that suchmodifications and alternatives are within the scope of this invention assubsequently claimed herein.

I claim:
 1. A method of optimizing packet length for wirelesstransmissions, comprising the steps of: selecting a trial packet lengthvalue; determining a packet success rate for the trial packet lengthvalue; evaluating a rate measure value for the trial packet lengthvalue, based upon the packet success rate for the trial packet lengthvalue; adjusting the trial packet length value responsive to adifference in the rate measure value for the trial packet length valuefrom a prior rate measure value for a prior trial packet length value;and repeating the selecting, determining, evaluating, and adjustingsteps until a convergence criterion is reached.
 2. The method of claim1, further comprising: selecting a learning constant value; wherein theadjusting step comprises: multiplying the difference in rate measurevalues by the learning constant value; and adjusting the trial packetvalue by an amount corresponding to the result of the multiplying step.3. The method of claim 1, wherein the step of determining a packetsuccess rate comprises: applying the trial packet length value to apacket success rate function.
 4. The method of claim 1, wherein the stepof determining a packet success rate comprises: measuring an actualpacket success rate for a plurality of transmitted packets having thetrial packet length value.
 5. The method of claim 1, wherein the ratemeasure value corresponds to a product of the packet success rate and aratio of the payload portion of each packet to an overall packet lengthincluding interpacket spacing.
 6. The method of claim 1, furthercomprising: estimating an expected packet error rate based upon asignal-to-noise ratio of a channel; evaluating an actual packet errorrate from the actual transmissions of packets over the channel; andcomparing the actual packet error rate to the expected packet errorrate; wherein the selecting, determining, evaluating, adjusting, andrepeating steps are performed responsive to the result of the comparingstep.
 7. The method of claim 6, wherein the selecting, determining,evaluating, adjusting, and repeating steps are performed responsive tothe comparing step determining that the actual packet error rate exceedsthe expected packet error rate by at least a selected threshold amount.8. A wireless local area network adapter, comprising: an interface, forcoupling the adapter to a host computer; radio circuitry, fortransmitting and receiving signals over an antenna, the transmittedsignals being in the form of packets including a payload portion and apreamble portion, with successive packets being transmitted with aselected interpacket spacing therebetween; signal processing circuitry,coupled between the interface and the radio circuitry, for modulatingsignals to be transmitted by the radio circuitry, and for demodulatingsignals received by the radio circuitry; and programmable logic, foroptimizing the length of the payload portion of the packets, programmedto optimize the length of the payload portion of the packets byexecuting a sequence of operations comprising: selecting a trial packetlength value; determining a packet success rate for the trial packetlength value; evaluating a rate measure value for the trial packetlength value, based upon the packet success rate for the trial packetlength value; adjusting the trial packet length value responsive to adifference in the rate measure value for the trial packet length valuefrom a prior rate measure value for a prior trial packet length value;repeating the selecting, determining, evaluating, and adjusting stepsuntil a convergence criterion is reached; and then setting the length ofthe payload portion of each of the packets according to the trial packetlength value upon the convergence criterion being reached.
 9. Theadapter of claim 8, wherein the sequence of operations performed by theprogrammable logic further comprises: selecting a learning constantvalue; wherein the adjusting operation comprises the steps of:multiplying the difference in rate measure values by the learningconstant value; and adjusting the trial packet value by an amountcorresponding to the result of the multiplying step.
 10. The adapter ofclaim 8, wherein the operation of determining a packet success ratecomprises the step of: applying the trial packet length value to apacket success rate function.
 11. The adapter of claim 8, wherein theoperation of determining a packet success rate comprises the step of:measuring an actual packet success rate for a plurality of transmittedpackets having the trial packet length value.
 12. The adapter of claim8, wherein the rate measure value corresponds to a product of the packetsuccess rate and a ratio of the payload portion of each packet to anoverall packet length including interpacket spacing.
 13. The adapter ofclaim 8, wherein the sequence of operations performed by theprogrammable logic further comprises: estimating an expected packeterror rate based upon a signal-to-noise ratio of a channel; evaluatingan actual packet error rate from the actual transmissions of packetsover the channel; and comparing the actual packet error rate to theexpected packet error rate; wherein the sequence of the selecting,determining, evaluating, adjusting, and repeating operations is executedresponsive to the result of the comparing step.
 14. The adapter of claim13, wherein the sequence of the selecting, determining, evaluating,adjusting, and repeating operations is executed responsive to thecomparing operation determining that the actual packet error rateexceeds the expected packet error rate by at least a selected thresholdamount.
 15. The adapter of claim 8, wherein the signal processingcircuitry comprises: a baseband processor, for modulating anddemodulating signals to be transmitted and received, respectively; andmedium access control circuitry, coupled to the interface, forprocessing signals to be modulated and received demodulating signals.16. The adapter of claim 15, wherein the programmable logic comprises anembedded central processing unit; and wherein the embedded centralprocessing unit, the baseband processor, and the medium access controlcircuitry are integrated into a single integrated circuit.
 17. A methodof controlling the fragmentation of packets to be transmitted over awireless communications channel, comprising the steps of: estimating anexpected packet error rate based upon a signal-to-noise ratio of achannel; evaluating an actual packet error rate from the actualtransmissions of packets over the channel; and comparing the actualpacket error rate to the expected packet error rate; wherein theselecting, determining, evaluating, adjusting, and repeating steps areperformed responsive to the result of the comparing step.
 18. The methodof claim 17, wherein the selecting, determining, evaluating, adjusting,and repeating steps are performed responsive to the comparing stepdetermining that the actual packet error rate exceeds the expectedpacket error rate by at least a selected threshold amount.